Preheat the oven to 200°C / fan 180°C. Generously butter a baking dish (24-26cm in diameter) - then sprinkle liberally with cassonade/demerara sugar.
In a large bowl, whisk together the caster sugar, vanilla sugar, salt, and plain flour.
In a jug, weigh out the eggs, milk, and cream. Gradually pour the milk and cream mixture into the dry ingredients, whisking just enough to just bring it all together. Stir in the melted butter.
Arrange the plums in the prepared dish, cut-side down, and pour the batter over them gently.
Bake for 30 minutes, then reduce the oven temperature to 180°C / fan 160°C and bake for another 20-30 minutes, or until golden brown and set - with the center still slightly wobbly.
Let it cool for a moment before serving - warm or at room temperature.